Synesthesia is a rare condition (not disorder—there is nothing unhealthy about it) in which a person’s neurological signals are mixed up in some way that researchers have not yet determined. Synthetes experience multiple sensations when most people would only experience one. For instance, a person with synesthesia might hear a musical instrument and see colors associated with it, or smell a food and see the smell as some sort of shape. One person I have read about sees orange when she feels pain; another feels sensations on his skin when he tastes food.
I experience it only mildly: I see the shapes of letters and numbers in certain colors. It’s more of a knowledge in my mind that a particular letter is a particular color as it is a physical seeing of the letter. This particular form of synesthesia is fairly common among synesthetes, though they rarely agree on what letters are what colors.
I’ve never met another synesthete, though, I think, and I’d like to.